The table on which a sacrifice is offered. In the Church founded by Christ, the altar is the table on which the Sacrifice of the Mass is offered. In ancient basilicas it was placed so that the priest faced the laity. Later, church altars were placed against or near the wall of the apse, so that the celebrant faced the east and the people were behind him, in the manner which now generally obtains. Altars of the early Church were probably of wood. Altars of stone and precious metals were introduced at a later date, and ecclesiastical law now stipulates that to be consecrated an altar must be of stone.

In the primitive Church two types of altars were used: the arcosolium or monumentum arcuatum, consisting of an archlike niche hewn in the catacombs over the grave of a martyr, which was covered by a slab of marble; and the detached altar found in the cubicula, or sepulchral chapels, formed by a slab of stone or marble resting on columns, or on a structure in which were enclosed the relics of martyrs. A decree of Saint Felix I stipulated that Mass should be celebrated on the tombs of martyrs. The tomb or chest type of altar thus replaced the simple table, and every altar must now contain the relics of martyrs. In the Greek Church, the altar proper is square, and the top should be constructed of wood, or have at least one board in it. Two coverings are used on it, one of linen, and the other of brocade or embroidery. The term altar is also applied to that part of churches of the Greek Rite practically corresponding to the sanctuary in churches of the Latin Rite, including the altar proper, a small side altar, the seats of the clergy, and the throne of the bishop.